PSI - Issue 17

D.G. Papageorgiou et al. / Procedia Structural Integrity 17 (2019) 532–538 D.G. Papageorgiou, H. Bravos, C. Medrea/ Structural Integrity Procedia 00 (2019) 000 – 000

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the first consideration of power generation producers (Huda et al. 2014). Systematic researches were done in emissions reduction and experts believe that there are still opportunities for even better efficiencies (Yokoyama and Ito 2006). The next decade, the number of projects involving combined cycle technology will increase by 3.1 % (Anver Comp. 2018) due to improved efficiency and environmental reasons (Balat 2010). A local company is currently producing electric power in a combined cycle power plant. Initially, the gas is delivered via pipelines to the preheating chamber and then is transferred by the same way to the gas turbine in order to be burned. The heat of the exhaust gases from the first engine is high enough that energy can be extracted from a subsequent steam turbine. The mechanical energy produced by the heat engines is driving electrical generators producing electricity. The superheated steam produced from the exhaust gases of the gas turbine is directed to a steam turbine, where it generates extra power. The exerted steam is condensed via an exchanger in order to return to the boiler and close the cycle. The circulating water in the exchanger is cold by a cooling tower. The cooling tower is comprised of series of roof fans which empower its cooling efficiency (Fig. 1a). The blades of the fans are 10m in length. The fan blade supporting assembly is frequently fails (arrows on Fig. 1b). A detailed study has been performed on the failure of the fan blade supporting system as the total breaking of the holders is very dangerous for the working personnel as well are also leading to increased downtimes for the power generating plant. Understanding the failure mechanism and solving the specific failure case is a matter of safety and an economic demand.

Fig. 1. (a) Detail of the cooling tower showing a part of the roof fans, (b) Image showing the fan blade support system.

2. Experimental

Recorded history was collected regarding usage and maintenance of the fans and their subassembly. Information for the manufacturing of the fan support was also documented. Three failed pieces were collected and analyzed in depth. Preliminary examination was implemented (Bravos et al. 2018). The pieces were optically inspected and a file of photographs was created, immediately after their collection. Accordingly, the surface of the U shape bolts was sandblasted. The component geometry and its fracture surfaces were examined again under stereoscope and by naked eye. Then samples from the holders for the analysis were selected. The pieces were firstly fragmented using a band saw and additionally cut using a cut off machine. Hardness measurements (HV30) were carried out across the diameter of the parts. Chemical analysis was performed using an optical emission spectrometer. The material selected for the manufacturing of the fan supports was identified. Selected specimens were accordingly prepared (sampling, mounding, grinding, polishing and etching) and metallographic analysis was conducted. The microstructure was examined near and far from the failed area. The cross section of the fracture surfaces were also in depth inspected.

3. Results and Discussions

The condensing unit is using twenty fans to the cooling tower. Each fan has seven blades which are situated concentrically to the center of a base. The blades are assembled with a pair of saddle-like aluminum receptors (upper and lower). The three-piece assemblement is then tightening up to the base via two U-bolt holders. They frequently break (Fig. 2a). Documents are not maintained regarding manufacturing, usage and maintenance of the parts. The holders are manufactured using hot rolled round bars. They are lathe-machined, U-shaped in bender and thread-